Annular concentric-lay bead cords are widely used as reinforcements for beads of various vehicle tires. As shown in FIG. 3, many of such cords are formed by helically wrapping a wrap wire 2 in a single layer or a plurality of layers around an annular core 1 formed by abutting and welding together both ends of a steel wire (as disclosed in Patent document 1). The wrap wire 2 has a smaller diameter than the annular core 1.
The steel wire forming the annular core is typically a mild steel wire containing carbon by 0.06 to 0.15 percent by weight. In order to ensure strength and rigidity sufficient to obtain formability necessary when wrapping a wrap wire, the annular core has a diameter not less than 1.5 times the diameter of the wrap wire. But in order to improve fuel economy of vehicles, a bead cord is acutely desired having a small-diameter annular core and thus lightweight, thereby reducing the weight of the tire in which the bead cord is embedded.
The wrap wire is a hard steel wire plated with brass or bronze to improve adhesion strength with the rubber of a tire. Plating is typically electroplating or displacement plating. The thickness of the plating is typically about 0.2 to 0.3 micrometers. The reason why the plating is this thin is because the thinner the plating, the higher the adhesion strength between the wire and the rubber. On the other hand, the annular core is often used in a bare state, so that it tends to corrode if the rubber becomes moistened. With the increasing requirements for higher standards of safety and high performance of tires, higher corrosion resistance is now required for the annular core too.
In order to answer these requirements, Patent publication 2 discloses a bead cord comprising an annular core and a wrap wire that is equal in diameter to the annular core and made of hard steel. In order to increase the adhesion strength between the core and the rubber, the core is also plated as with the wrap wire. The adjacent turns of the wrap wire are spaced more widely from each other than those of conventional bead wires so that rubber reaches the annular core and are adhered thereto.
Since the annular core of the bead cord disclosed in each of the examples of Patent document 2 is a hard steel wire (carbon content: 0.72 to 0.82%), even though its diameter is as small as the diameter of the wrap wire, the annular core is as high in strength and rigidity as conventional annular cores. The bead cord is thus extremely lightweight compared to conventional ones. Further, even if the rubber becomes moistened, the annular core is less likely to corrode by contact with the rubber.
However, when a hard steel wire containing not less than 0.57% of carbon is welded, a hard and brittle, martensitic structure tends to develop in its welded portions, so that such a hard steel wire tends to break at the welded portions if used with no post treatment. Thus, it is necessary to anneal such a bead cord after abutting and welding both ends of a hard steel wire into the annular core. This pushes up the cost. Also, a long time is needed to remove burrs after welding, which also pushes up the cost.
Further, because the annular core has a diameter equal to the diameter of the wrap wire, although the annular core is equivalent in strength and rigidity to conventional large-diameter annular cores, it is more difficult to wrap the wrap wire uniformly and evenly around such a small-diameter annular core. Moreover, while the wrap wire is being wrapped around such a small-diameter annular core, the annular core may be pulled radially outwardly by the wrap wire to such an extent that the annular core is partially located radially outside of the wrap wire. This deforms the cord (such that when the cord is placed on a flat surface, it partially separates from the flat surface).
Moreover, it is practically difficult to wrap the wrap wire around the annular core such that the adjacent turns of the wrap wire are uniformly separated from each other. Thus, rubber cannot uniformly cover the annular core. At portions where the annular core is not covered by the rubber, the steel wire forming the annular core tends to quickly grow rust because the thin plating formed on the annular core to improve adhesion strength with the rubber does not provide the annular core with sufficient corrosion resistance. Such rust on the annular core will eventually infiltrate into the rubber. In the case of a vehicle tire, if fretting occurs between the annular core and the wrap wire, their plating layers tend to be worn off in a short period of time, which reduces the corrosion resistance, thus increasing the corroded area. By repeating plating treatment, it is possible to increase the final thickness of the plating to a certain extent. But because electroplating and electroless plating are inherently not adapted for forming a thick plating layer, work efficiency is inferior, which increases the cost.
Patent document 3 discloses a bead cord including an annular core made of a high-performance synthetic resin to reduce the weight of the cord. Since the annular core is made of a resin, the core shows improved corrosion resistance. But since a high-performance material is used, such a cord is expensive. Storage and handling are also difficult. Annular cores made of nylon 6 or polyethylene naphthalate (PEN), as used in examples of Patent document 3, will be insufficient in rigidity as bead cords if their diameter is equal to the diameter of conventional annular cores. Thus, such cords are not practically usable.    Patent document 1: JP patent publication 3499261    Patent document 2: JP patent publication 05-163686    Patent document 3: JP patent publication 11-321247